Proximity switches are generally known in the art and have been widely applied to sense the position of a moving object in manufacturing processes. Such known proximity switches utilize an oscillator drive circuit in combination with an induction tank circuit. The tank circuit includes an induction coil as a means for sensing the presence of an object such as metal. The induction coil is constructed such that it generates a magnetic field in an area surrounding the coil. The magnetic field induces eddy currents in a conductive object which comes within the generated magnetic field. Such objects are known in the art as targets. Once a target comes within the magnetic field of the coil, energy is drawn from the induction coil. A typical induction proximity switch selects components of the oscillator and tank circuit to insure that oscillations occur when a target is absent from the magnetic field of the induction coil. When a target comes within the magnetic field of the induction coil, the oscillation amplitude is attenuated due to the loss of energy caused by eddy currents induced in the target. The amount of the oscillation attenuation is directly related to the distance between the target and the induction coil.
A predetermined distance between the induction coil and the target is selected as the point where the output of the proximity switch changes an electrical state to indicate the presence of a target. This distance is known as the switch trip point. A circuit within the proximity switch monitors the oscillation amplitude and generates a signal at the output of the proximity switch indicative of the fact that the target has come within the trip point distance.
Most inductive proximity switches employ a ferrite cup core and coil assembly as the transducer or sensing element. This is connected to an oscillator, usually operating between 100 and 600 KHz. The frequency is determined by the resonant frequency of the coil and a high quality tuning capacitor. A cup core is preferred because it allows for the flux field to be focused in front of the cup core, allowing a further sensing distance. The cup core, however, is a liability when used in high magnetic field environments, such as in or near the throat of a welder. The ferrite material saturates in these fields, causing the oscillator to damp because of the resulting tank Q degradation. This action makes the oscillator act as though a target is present. Prior art attempts to overcome this problem have employed innovative oscillator designs and oscillator detector and hold circuitry. This approach has been partially successfully for the reason that the welding field is sinusoidal in nature and there is a time in every half cycle when the field is passing through zero. During this period, the oscillator is designed to rapidly build up and the detector responds rapidly, holding the detected voltage during the time the magnetic field builds up and damps the oscillator. While this can be accomplished satisfactorily, this type of oscillator is inherently more complicated and costly. In addition, there is still an upper limit as to the strength of magnetic field to which the current can be made immune.
There is presently an acute need for proximity switches which can be used in conjunction with pneumatic or hydraulic cylinders in automated manufacturing to detect when a host cylinder has reached either limit of travel. In automated manufacturing applications, particularly ganged welding of work pieces, numerous power actuated devices are employed, the actuation of which must be carefully choreographed for efficient work piece flow. Typically, separate proximity switches have been used for each actuator to separately detect each limit of travel. Because of the large number of proximity switches required, low cost and simplicity of design are paramount.
Another problem with prior art proximity switches lies in the fact that in adapting them for use with high pressure hydraulic cylinders, expensive and time consuming modification to the cylinders themselves must be made.
Still another problem of prior art proximity switches lies in the fact that the commercially available switch designs will not function properly when in close proximity to the extremely high electromagnetic fields generated by industrial welding operations. Shielding such proximity switches from external sources of electromagnetic energy has proven to be only partially effective and expensive.
A final problem with prior art proximity switches lies in the inherent difficulty in providing a reliable mechanism for positioning and aligning the fixed coil with the moving target. Because any variations in the path of travel of the target or changes in distances between target end of travel points and the coil can alter the proximity switch set point, cycle to cycle repeatability as well as long term stability are critical. Such problems are exacerbated by the presence of large electromagnetic fields because the coil and target must be confined within a shielded environment but electrically and magnetically isolated from the shielding itself.